This invention relates generally to an improved control system and process for three-phase AC motors used in such variable speed, load-carrying applications as elevators.
Prior art systems typically use a single winding AC motor system as shown schematically in FIG. 1. Clockwise and counterclockwise SCR banks 3 and 4 control the application of current to the three winding portions T1, T2 and T3 of motor 7.
One problem encountered in such prior are systems has been that of electrical short circuiting between the clockwise and counterclockwise SCR banks. If, for example, the C-phase of the clockwise SCR bank is energized at the same time as the B-phase of the counterclockwise SCR bank, as shown at numeral 5 of FIG. 1, a dangerous and potentially damaging short circuit condition occurs.
In order to avoid this condition, prior art systems have used a complex control panel 8 upstream of the SCR banks. The control panel typically has such items as rapid circuit breakers, phase sequence detectors, lock-out circuits, air chokes and current feedback devices to avoid and/or prevent damage from a line-to-line electrical short condition. Such systems are not only initially expensive but also require an inordinate amount of repair and maintenance costs.
Typically, such systems further require extensive use of an external mechanical brake when the load must be stopped or floated at an intermediate point of travel.
It is an object of the present invention to eliminate the need for such costly, complex and dangerous prior art systems by the use of two separate AC motor windings which are controlled in a novel manner as compared to known plural winding designs.